1. Field of the Invention
This invention relates to a method to determine carbon dioxide (CO2) leaks from sequestration reservoirs, and, more specifically, this invention relates to an improved and enhanced method for the detection and quantification of leaks of carbon dioxide from geologic formations.
2. Background of the Invention
Carbon dioxide is a greenhouse gas, yet is useful for the manufacture of such chemicals as carbonates. Sequestration of CO2, produced from fossil fuel-fired electric power generating facilities and other stationary sources is an essential part of this country's efforts to reduce greenhouse gas emissions. In June 2003 the United States signed a charter with the European Union's Executive Commission and twelve countries including Russia, China, Japan, Canada, and Brazil to research the technology for storage of carbon dioxide in geologic formations.
Approximately 7.4 Giga tons (Gt) of carbon dioxide are emitted annually from all fossil fuel emission sources such as automobiles, powerplants, and industrial facilities. About 25% of the 7.4 Gt are sequestered by terrestrial ecosystems, e.g., forests and agricultural plants. Less than 0.01% of the 7.4 Gt is intentionally sequestered.
United States policy may mandate carbon dioxide sequestration in geologic repositories. Exemplary repositories include active and depleted oil and gas wells, saline aquifers, salt domes, and deep, often unmineable coal deposits. Potential leaks from such repositories must be monitored.
The problem of substantiating, monitoring, and verifying the long-term storage integrity of carbon dioxide sequestered in underground reservoirs is complicated by the fact that the reservoirs may be very deep, sometimes greater than 5000 feet. Many reservoirs are covered by seemingly impenetrable rock strata. While many geologists believe that reservoirs are permanently sealed by cap rock strata, evidence exists that shows micro seepage can be a long-term factor. Accordingly, a need exists to determine if the carbon dioxide is effectively sequestered or moving vertically through or around a cap rock or laterally along a cap rock. This determination will help verify the integrity of physical barriers to the escape of carbon dioxide.
Direct determination of carbon dioxide leakage from the sequestration reservoir to the atmosphere is rendered difficult by the presence of atmospheric carbon dioxide. Further, atmospheric carbon dioxide's concentration is high and seasonally variable. The high concentration makes difficult the detection of micro seepage of carbon dioxide by simple measurement of carbon dioxide in the atmosphere above the sequestration reservoir. The seasonal variation would require that baseline determinations of atmospheric carbon dioxide concentration be frequently made.
In addition, carbon dioxide is often pumped into oil reservoirs to enhance oil recovery (EOR) and well yield. A method that can monitor carbon dioxide movement could provide information about the underground flow of fluids to improve such oil recovery techniques.
Further, some leaks can be intermittent. A breach of the natural seal between the storage area and higher strata can occur. That breach may be only temporary. For example, a certain quantity of CO2 may be released until the pressure on the overburden strata is reduced and the sequestration reservoir reseals itself. Pressure buildup and release may be a sporadic process, and any tracers used could be quickly dispersed. Continuous monitoring using ground-based systems would offer the ability to detect even sporadic intermittent leaks.
Carbon dioxide, when pumped into a geological formation, causes gases such as methane (CH4), ethane (C2H6), propane (C3H8), and radon (Rn) to desorb from the formation. Experience at EOR sites shows that concentrations of these gases immediately above reservoirs are known to respond quickly to well operations such as pumping rates, reservoir flooding, and over-pressurization of the reservoir. As such, increased surface flux and/or concentrations of these gases in soil gas can be monitored as an indicator of carbon dioxide leakage. Measurement of light hydrocarbon concentrations in the soil above a sequestration reservoir as a means for monitoring carbon dioxide leakage has been proposed. This is discussed in: R. W. Klusman, “Evaluation of Leakage Potential from a Carbon Dioxide EOR/Sequestration Project,” Energy Conversion and Management, 44, 1921-1940 (2003).
In any reservoir leak scenario, carbon dioxide displaces methane and other gases from any geological media onto which the methane and other compounds are sorbed. Thus, methane moves ahead of carbon dioxide. In a subsurface environment, methane, ethane, propane, and radon are more mobile because they are less adsorptive and less water-soluble than carbon dioxide. Increased amounts of methane, other light hydrocarbons, and Rn at or near the surface of a sequestration reservoir suggests a leak pathway from the reservoir to the surface. Sequestration into that particular geological formation should then be reconsidered. If methane and other light hydrocarbons from the sequestration formation can leak to the surface, so can carbon dioxide.
In the alternative, tracer molecules can be injected with the carbon dioxide into sequestration reservoirs. Radioactive tracers have been used extensively in the past, but regulation and public opinion have discouraged their use. Halogen- and/or sulfur-containing compounds can serve well as tracers. These compounds have very low background levels in the atmosphere, and can be determined at low levels using conventional gas chromatographic analysis with electron capture or photo-ionization detectors, or combined gas chromatography-mass spectrometry.
Tracers should have a high level of detectability, be free from significant background interference, have a similarity to carbon dioxide in migration and chemical characteristics, be chemically stable, be safe and environmentally benign, and be relatively non-corrosive. Also, tracer cost in bulk will be an important consideration inasmuch large amounts of the tracers may be needed to spike an underground reservoir. Several tracers can be employed simultaneously, each with its own unique advantages and limitations.
Perfluorocarbon (PFTs) tracers have been tested and shown to be an effective means to evaluate barrier integrity, as shown in T. M. Sullivan, J. Heiser, A. Gard, and G. Senum, “Monitoring Subsurface Barrier Integrity Using Perfluorocarbon Tracers,” Journal of Environmental Engineering, 124 (6) (June) 490-497 (1998).
PFTs have been used to locate and quantify leaks in water cooled stator bars in nuclear power plant generators. Conventional detection methods for PFTs include collection of the leaking PFTs on sorbents with subsequent gas chromatographic-electron capture detection.
U.S. Pat. No. RE36,951 awarded to Cooper, et al. on Nov. 14, 2000 discloses the use of a leak detection dye in climate control systems.
U.S. Pat. Nos. 6,070,455 and 5,681,984 awarded to Cavestri on Jun. 6, 2000, and Oct. 28, 1997, respectively, disclose the use of a leak detecting dye in climate control systems. The dye is absorbed onto a non-absorbent carrier.
U.S. Pat. No. 5,143,568 awarded to Sheahan on Sep. 1, 1992 discloses a leak detection method using a tracer gas to test the integrity of roofing seams.
U.S. Pat. No. 4,755,757 awarded to Cooper on Jul. 5, 1988 discloses a fluid leak detection system for determining the rate of fluid leakage through a geomembrane.
None of the aforementioned patents disclose a method for detection of carbon dioxide leakage from a sequestration reservoir. In particular, a method to account for background/soil/bacteria sources of carbon dioxide is not disclosed.
A need exists in the art for a method of detection of carbon dioxide leakage from sequestration reservoirs. The method should account for background carbon dioxide sources when engaging in carbon dioxide leak detection from the reservoirs. The method should also be environmentally benign, and be economical in use. The method should also provide a means for branding the injected CO2 so as to identify the source/ownership of the sequestered CO2.